A. Technical Field
The present invention relates to: a (meth)acrylic acid (salt) polymer which is used favorably for such as cement dispersants and pigment dispersants; and its production process.
B. Background Art
Poly(carboxylic acids), which are obtained by a process including the step of copolymerization of a monomer component including an unsaturated monocarboxylic monomer (a) and an unsaturated polyalkylene glycolic monomer (b) as essential components, have hitherto been used favorably for cement dispersants and pigment dispersants, and besides, various other dispersants.
For example, in JP-A-018338/1984, the poly(carboxylic acids) are produced from a polyalkylene glycol mono(meth)acrylate ester monomer, a (meth)acrylic monomer, and another monomer, copolymerizable with these monomers, using a peroxide (e.g. benzoyl peroxide, lauroyl peroxide), a hydroperoxide (e.g. cumene hydroperoxide), or an aliphatic azo compound (e.g. azobisisobutyronitrile) as a polymerization initiator.
However, as to such a reaction system between the unsaturated monocarboxylic monomer (a) and the unsaturated polyalkylene glycolic monomer (b), in the case where this reaction is carried out at a high temperature, the polymerization conversion of the monomers is high. However, there occur problems such that: the amount of an oligomer having a low molecular weight as formed tends to increase so much as to decrease an effective polymer portion having such a high molecular weight as to be able to display performance as a dispersant. On the other hand, in the case where the reaction is carried out at a low temperature, the chain transfer of the monomers can be suppressed, and therefore there is obtained a (meth)acrylic acid (salt) polymer in which the amount of oligomer portions is small and the amount of effective polymer portions is large. However, the polymerization conversion of the monomers tends to be low.
In order to solve such problems, there may be conceived an idea of, in a process for production of the aforementioned (meth)acrylic acid (salt) polymer, carrying out the reaction at a low temperature, and prolonging the reaction time in order to raise the polymerization conversion of the monomers. However, a long time is needed for the reaction, and therefore the productivity and workability are lowered and costs are problematic when the reaction is industrially carried out.
Thus, as to the aforementioned reaction system, it is desirable to use a polymerization initiator having high reactivity even at a low temperature.
Examples of polymerization initiators for meeting such a demand include persulfate salts, such as ammonium persulfate and sodium persulfate.
As one of publicly known techniques in which the persulfate salt is used as an initiator in the aforementioned reaction system, there is a method as disclosed in WO 01/14438. In this method, arrangements are made to obtain a (meth)acrylic acid (salt) polymer having a good property (paste flow value which indicates cement dispersibility) by adjusting the pH of the polymerization reaction liquid into the range of 1.5 to 3.5. In addition, in this prior technique, arrangements are made to produce the unsaturated polyalkylene glycolic monomer (b) (which is one of the monomers) by carrying out esterification of a polyalkylene glycol monoalkyl ether and (meth)acrylic acid as raw materials with an acid catalyst, and to deactivate the above acid catalyst with an alkali agent after the end of the esterification, thus smoothing the polymerization.
However, according to the present inventors' study, the above prior method as disclosed in WO 01/14438 causes the following problems because of using the persulfate salts (e.g. ammonium persulfate) as the initiators without devising anything. That is to say, in the cases of these initiators, sulfuric acid is generated during the polymerization. Therefore, a stainless-steel-made reaction vessel may be corroded, so it is necessary to use a glass-lined reaction vessel. The glass-lined reaction vessel has a lower thermal conductivity than the stainless-steel-made reaction vessel, and is low efficient in heating and cooling, and besides, it is weak against impact, and its repair is also difficult in the case where the lining layer has cracked. Therefore, there is a problem such that much expense is necessary for the maintenance of facilities. In the case where the poly(carboxylic acids) are used as cement dispersants, it is usually desirable, for their easy handling, to neutralize them after the polymerization. However, as to the (meth)acrylic acid (salt) polymer as obtained by using the persulfate salt as the initiator, a salt easily forms during the neutralization, so there is a possibility that various troubles may be caused by deposit of this salt as its crystals. In addition, as to an aqueous solution of the (meth)acrylic acid (salt) polymer as obtained by using the persulfate salt as the initiator, there are cases where this aqueous solution is colored dark brown. Therefore, this aqueous solution is unsuitable for use as a pigment dispersant in which there is a high demand for dispersants which are little colored.
According to the present inventors' study, the above prior method as disclosed in WO 01/14438 is also a method such that the polymerization conversion of the monomers is still not sufficient, so a portion of the monomers easily remains unreacted. Therefore, as to the (meth)acrylic acid (salt) polymer as produced by the above prior method, there are problems such that its high combining ratio is involved in composing a dispersant.